Polyethylene-Syndiotactic Polypropylene Composition and Processing Thereof

ABSTRACT

A processable polypropylene composition comprising a mixture of a major amount of a syndiotactic polypropylene and a minor amount of polyethylene wax. The syndiotactic polypropylene has a designated recrystallization temperature, an original minor melting peak and a higher original melting peak. The polyethylene wax provides a recrystallization temperature of the mixture that is greater than the recrystallization temperature of the syndiotactic polypropylene alone and also provides for a minor melting peak and a major melting peak of the mixture, which has a temperature difference which is lower than the difference between the original minor melting peak and the original major melting peak of the syndiotactic polypropylene alone. A process for the production of polypropylene product employing a polyethylene-syndiotactic polypropylene composition as described which is heated to provide a plastic mass of the syndiotactic polypropylene and polyethylene which is processed to provide the product.

FIELD OF THE INVENTION

This invention relates to syndiotactic polypropylenes incorporatingsmall amounts of polyethylene waxes and methods of processing suchpolypropylenes to produce a polypropylene product.

Stereoregular polypropylene in the form of propylene homopolymers orpropylene co-polymers incorporating small amounts of other alpha olefinssuch as ethylene or butylene as co-monomers can be processed to formproducts such as films, fibers and molded products. Of the stereoregularpolypropylenes, perhaps isotactic polypropylene is most widely used. Forexample, isotactic polypropylene can be used in the production ofrelatively thin films in which the polypropylene is heated and thenextruded through slotted dies and subject to biaxial orientation bystretching the film in both a longitudinal and a transverse direction.Isotactic polypropylene can also be extruded through small orifices toproduce fibers or may be subjected to blow molding or injected intoconstriction molds to produce molded products such as containers andsolid shaped items.

The structure of isotactic polypropylene is characterized in terms ofthe methyl group attached to the tertiary carbon atoms of the successivepropylene monomer units lying on the same side of the main chain of thepolymer. That is, the methyl groups are characterized as being all aboveor below the polymer chain. Isotactic polypropylene can be illustratedby the following chemical formula:

Stereoregular polymers, such as isotactic and syndiotacticpolypropylene, can be characterized in terms of the Fisher projectionformula. Using the Fisher projection formula, the stereochemicalsequence of isotactic polypropylene as shown by Formula (1) can becharacterized as follows:

Another way of describing the structure is through the use of NMR.Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . .with each “m” representing a “meso” dyad, or successive methyl groups onthe same side of the plane of the polymer chain. As is known in the art,any deviation or inversion in the structure of the chain lowers thedegree of isotacticity and crystallinity of the polymer.

In contrast to the isotactic structure, syndiotactic propylene polymersare those in which the methyl groups attached to the tertiary carbonatoms of successive monomeric units in the polymer chain lie onalternate sides of the plane of the polymer. Using the Fisher projectionformula, the structure of syndiotactic polypropylene can be shown asfollows:

Syndiotacticity can be characterized in terms of the syndiotactic dyadand pentad rrrr in which each “r” represents a racemic dyad.Syndiotactic polymers are semi-crystalline and, like the isotacticpolymers, are essentially insoluble in xylene. This crystallinitydistinguishes both syndiotactic and isotactic polymers from an atacticpolymer, which is non-crystalline and highly soluble in xylene. Anatactic polymer exhibits no regular order of repeating unitconfigurations in the polymer chain and forms essentially a waxyproduct.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided aprocessable polypropylene composition comprising a mixture of asyndiotactic polypropylene as the predominant component and apolyethylene wax as a relatively minor component. The syndiotacticpolypropylene is characterized by a designated recrystallizationtemperature, an original minor melting peak, and an original majormelting peak occurring at a higher temperature than the original minormelting peak. The polyethylene wax is present in an admixture with thesyndiotactic polypropylene in an amount to provide a recrystallizationtemperature of the admixture that is greater than the recrystallizationtemperature of the syndiotactic polypropylene alone. The polyethylenewax also provides for a minor melting peak and a major melting peak ofthe mixture, which has a temperature difference which is lower than thetemperature difference between the original minor melting peak and theoriginal major melting peak of the syndiotactic polypropylene alone. Inan embodiment, the temperature difference between the minor and majormelting peaks of the formulation incorporating the polyethylene wax isless than the difference between the original minor and major meltingpeaks, that is, without the addition of the polyethylene wax, by anincrement of at least 2° C. In an embodiment, the polyethylene wax,which has a molecular weight that is less than the molecular weight ofthe syndiotactic polypropylene, is present in an amount to provide anenhancement in the recrystallization temperature that is at least 5%greater than the recrystallization temperature of the syndiotacticpolypropylene alone.

The processable polypropylene composition of the present invention isalso characterized in terms of the slope of the curve defined by theoriginal minor melting peak of the syndiotactic polypropylene alone andthe minor melting peak of the admixture of polyethylene wax and thesyndiotactic polypropylene. This slope is greater than the slope of thecurve defined by the original major melting peak of the syndiotacticpolypropylene and the major melting peak of the admixture ofsyndiotactic polypropylene and the polyethylene wax.

In an embodiment of the invention, the processable composition ischaracterized in terms of the temperature difference between therecrystallization temperature of the mixture and the minor melting peakof the mixture, which is less than the difference between the originalrecrystallization temperature and the original minor melting peak by anincrement of at least 1° C. In another embodiment, the polyethylene waxis present in the admixture in an amount of at least 0.1 wt. % andranging up to 10 weight percent to provide an amount within the range of0.1-10 wt. %.

In another embodiment of the invention, there is provided a process forthe production of a polypropylene product employing apolyethylene-syndiotactic polypropylene composition as described above.The polyethylene-syndiotactic polypropylene composition is heated to atemperature in excess of the major melting peak of the composition toprovide a plastic mass of the syndiotactic polypropylene incorporating asmall amount of polyethylene. This plastic mass is processed to adesired configuration to provide a preform for the product and thencooled to a temperature below the recrystallization temperature of theadmixture to provide the product. The plastic mass may be processed byextruding it through a slotted die system to provide a polypropylenefilm. In another embodiment of the invention, the plastic mass may beblow molded or introduced into a confining mold and then cooled to atemperature below the recrystallization temperature to produce a moldedproduct, or the plastic mass may be extruded through a fiber die orificeto produce an elongated fiber preform. The preform is then cooled to atemperature below the recrystallization temperature and then stressed byelongation to form the final fiber product.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical illustration of increase in recrystallizationtemperature and decrease in incremental difference between minor andmajor melting peaks as a function of polyethylene wax concentration insyndiotactic polypropylene; and

FIG. 2 is a graphical illustration of the major melting peak and theminor melting peak as a function of the polyethylene wax concentrationin the syndiotactic polypropylene mixture.

DETAILED DESCRIPTION OF THE INVENTION

The syndiotactic polypropylene employed in carrying out the presentinvention can be characterized in terms of the microcrystallinecharacteristics and the rheological properties that are significant inthe processability of the polymer product. The microstructure andrheological products of syndiotactic polypropylene are described in U.S.Pat. No. 6,184,326 to Razavi et al. As disclosed there, syndiotacticpolypropylene can be characterized in terms of its molecular weight andmolecular weight distribution, M_(w)/M_(n), as defined by the ratio ofthe weight average molecular weight, M_(w), to the number averagemolecular weight, M_(n), and it microstructure in terms of the percentof racemic dyads, indicated by r, racemic triads, indicated by rr, andracemic pentads, indicated by rrrr. As further described in the patentto Razavi et al., propylene can be polymerized to produce syndiotacticpolypropylene to peak temperatures characterized in terms of a lowermelting point, identified as Tm1, and a higher melting point, identifiedas Tm2 in the Razavi patent. For a further description of syndiotacticpolypropylene, its microstructure, rheologic characteristics and meltingpoint characteristics, reference is made to the aforementioned U.S. Pat.No. 6,184,326, the entire disclosure of which is incorporated herein byreference.

The syndiotactic polypropylene employed in the present invention mayhave a relatively high syndiotacticity as measured by the percent of theracemic pentads of the syndiotactic polypropylene. In an embodiment, themicrostructure is characterized by at least 70% racemic pentads, andmore specifically at least 75% racemic pentads and further at least 80%racemic pentads. The molecular weight of the syndiotactic polypropyleneemployed in the present invention can be of any suitable value, but itnormally will be substantially higher than the polyethylene wax that ismixed with the syndiotactic polypropylene. The molecular weightdistribution may be broad or narrow, depending upon the ultimate use towhich the polymer is put. The molecular weight distribution can becontrolled by selection of the particular transition metal used in thesyndiospecific catalyst or through the use of two differentsyndiospecific metallocenes as disclosed in the aforementioned U.S. Pat.No. 6,184,326 to Razavi et al.

The polyethylene wax incorporated into the syndiotactic polypropylenenormally will be present in an amount of at least 0.1 wt. %, andalternatively in an amount of at least 1 wt. %. The polyethylene waxconcentration may vary up to a value of about 10 wt. % of thesyndiotactic polypropylene, but normally will be used in somewhat loweramounts of about 1 wt. %. As described previously and as shown by thefollowing experimental work, the amount of syndiotactic polypropylenecan be varied in order to produce a desired rheological characteristicof the syndiotactic polypropylene-polyethylene wax mixture. Thepolyethylene wax as described previously will normally have asubstantially lower molecular weight than the molecular weight of thesyndiotactic polypropylene. Normally the molecular weight of thepolyethylene wax will be no more than 5,000. Usually the molecularweight will vary within the range of 1,000-3,000.

In processing the syndiotactic polypropylene-polyethylene wax mixture,the solid polymer product, as it is progressively heated, reaches afirst lower temperature melting peak, characterized herein as the minormelting peak, and upon further heating reaches a second, highertemperature melting peak, characterized herein as the major meltingpeak. As the molten polymer product is then cooled, it reaches arecrystallization peak that is at a temperature significantly below thetemperature of the minor melting peak. In the present invention, theprocessing of the syndiotactic polypropylene-polyethylene wax mixturecan be controlled and enhanced by bringing the minor and major meltingpeaks in closer proximity to one another than observed for the puresyndiotactic polypropylene and also by increasing the recrystallizationpeak to a value greater than that observed for the original or puresyndiotactic polypropylene, that is, the polypropylene without theaddition of the polyethylene wax.

In experimental work respecting the invention, four samples of asyndiotactic polypropylene were tested with respect to their theologicalproperties as characterized by their minor and major melting peaks andtheir recrystallization temperatures. The three samples incorporated apolyethylene wax in various concentrations, and one sample comprisedonly the syndiotactic polypropylene without the polyethylene waxaddition.

The syndiotactic polypropylene employed in the experimental work washighly crystalline, having >90% syndiotactic dyads and >75% syndiotacticpentads. The syndiotactic polypropylene was produced by polymerizationwith a syndiospecific metallocene of the type disclosed in theaforementioned U.S. Pat. No. 6,184,326. The polymer had a weight averagemolecular weight of about 170,000 and a number average molecular weightof about 35,000, providing a molecular weight distribution of 4.8.

The polyethylene wax employed in various concentrations in admixturewith the syndiotactic polypropylene had a molecular weight of about3,000 and a melting temperature of about 128° C. The polyethylene waxwas employed in compositions varying from 0.1 wt. % polyethylene wax to5.0 wt. % polyethylene wax. The percentage of polyethylene wax isdetermined based upon the mixture of the polyethylene and syndiotacticpolypropylene. Thus, the formulation containing 5.0 wt % polyethyleneincorporated 5 parts by weight of polyethylene and 95 parts by weight ofsyndiotactic polypropylene.

The polyethylene wax employed was employed in two forms. One was amicronized form having an average particle size of 7 microns, and theother in the form of prills averaging 200 micron particles. The puresyndiotactic polypropylene and the polyethylene-syndiotacticpolypropylene formulations were formed into plaques having a thicknessof 0.05″. The characteristics of the formulations and the plaques thusformed are set forth in Table 1. In Table I, the four samples used areidentified as samples 1, 2, 3 and 4, with sample I being the purepolypropylene. Column no. 1 indicates the concentration of polyethylenewax and column no. 2 indicates the nature of the wax used in the samplein question. In the third column, the haze of the 0.05 inch plaques isset forth. The haze factor was determined according to ASTM Method D1003(Procedure A). The fourth column sets forth the relative injection speedin forming the plaques and the fifth column indicates the cooling timefor the plaques in seconds. The sixth column shows the total cycle timefor producing the plaques in seconds.

Table II presents the rheological properties of the four samplesidentified in Table I. In Table II, the recrystallization temperature,T_(r), and the major, T₁, and minor, T₂ temperature peaks in ° C. areshown respectively in the first three rows of data. The fourth rowindicates the difference between the major (T₁) and the minor (T₂)melting peaks for each of the samples 1, 2, 3 and 4. The fifth rowindicates the difference between T₂ and T_(r) and the last row indicatesthe difference between T₁ and T_(r).

TABLE I Sample 1 2 3 4 5 6 1 9.5 75 60 81.5 2 0.1% micronized, avg. 713.0 69 80 103.3 micron particles 3 1.0% micronized, avg. 7 9.4 69 80101.6 micron particles 4 5.0% prill, avg. 200 63.7 69 60 81.6 micronparticles

TABLE II 1 2 3 4 T_(r) 69.9 70.4 75.3 82.8 T₁ 129.5 128.5 129.1 129.1 T₂116.6 115.5 118.3 122.8 T₁ − T₂ 12.9 13 10.8 6.3 T₂ − T_(r) 46.7 45.1 4340 T₁ − T_(r) 59.6 58.1 53.8 46.3

Certain of the data set forth in Table II are shown graphically in FIGS.1 and 2. In FIG. 1, curve 2 is a plot of the incremental difference Dbetween the major and minor melting peaks (T₁-T₂) in ° C. plotted on theordinate vs. the polyethylene concentration, P, plotted on the abscissaand curve 3 is a graph of the increase in recrystallization temperatureIT, from the value of T_(r) for sample 1 plotted on the ordinate vs. thepolyethylene concentration, P.

In FIG. 2, curve 4 is a plot of the major melting peak, T₁, plotted onthe ordinate vs. the polyethylene concentration, P, plotted on theabscissa. Curve 6 is a plot of the minor melting peak, T₂, plotted onthe ordinate vs. the polyethylene concentration, P, plotted on theabscissa. In reviewing the experimental work, it appears that theinitial slight decrease in T₁ and T₂ may be an anomaly resulting fromexperimental error and accordingly, curves 4 and 6 in the graph aredrawn through an average of the data for the syndiotactic polypropylenealone and the syndiotactic polypropylene containing 0.1 wt. %polyethylene.

As can be seen from examination of the above-referenced experimentalwork, the addition of even a small amount of polyethylene wax hasseveral beneficial results in terms of enhancing processability of thesyndiotactic polypropylene during formation of the initial preform andduring processing to produce the desired product. A significant increasein recrystallization temperature is observed, along with an increase inthe minor melting peak. The major melting peak remains more or lessflat, with an increase in polyethylene concentration so that the twomelting peaks tend to merge, enhancing processability of thesyndiotactic polypropylene during formation of the preform. At least 1wt. % polyethylene wax may be employed in order to achieve a significantincrease in recrystallization peak along with substantial merging of themajor and minor melting peaks. As indicated by the data shown in FIG. 1where curves 2 and 3 reach a crossover point near 2 wt. %, at least 2wt. % polyethylene wax is employed in order to provide a mixture at orabove this crossover point.

As shown by the data in FIG. 2, the increase in the minor melting peak,as indicated by curve 6, continues through the 5 wt. % polyethyleneformulation, albeit with a decreasing slope. Accordingly, while greateramounts of polyethylene can be employed, up to 10 wt. %, thepolyethylene content may be limited to 5 wt. %, and alternativelylimited to 4 wt. % in order to prevent further degradation of clarity ofthe product as indicated by the increased haze factor shown in Table I.In FIG. 2, broken lines 7 and 8 indicate the slope of the curve definedby the original minor melting peak of the syndiotactic polypropylenealone and the peaks as observed at 2 and 5 wt. %, respectively. In bothcases, they are substantially greater than the slope of curve 4, whichis relatively flat.

The syndiotactic polypropylene-polyethylene wax mixture may be employedin accordance with the present invention to produce any suitableproduct. Thus, the invention may be carried out to produce fibers,monolayer or multilayer films and molded products in either blow moldingprocesses or constrictive molding processes. In each case, thepolyethylene-syndiotactic polypropylene composition is heated to atemperature in excess of the major melting peak to produce a molten orplastic mass of the composition, which can be further processed to adesired configuration. The preform thus produced is then cooled to atemperature below the recrystallization temperature to provide thedesired product. This product may be further processed in a manner knownto those skilled in the art in order to arrive at the desired product.

The present invention is particularly applicable to the production ofpolypropylene films and fibers. Suitable procedures for producing fibersformed of the polyethylene wax-syndiotactic polypropylene composition ofthe present invention are disclosed in U.S. Pat. No. 6,416,699 toGownder. As disclosed in Gownder, the syndiotactic polypropyleneformulation is heated to a molten state, in this case in excess of themajor melting temperature peak, and then extruded to form a fiberpreform which is spun in a quench column and then drawn at any suitabledraw ratio, typically within the range of 2-3.

Where the present invention is employed in the production ofpolypropylene films, the molten mass, again heated to a temperature inexcess of the major melting point peak, is extruded through a slotteddie configuration and then processed, for example, in a suitable tenterframe procedure as disclosed in U.S. Pat. No. 6,641,913 to Hanyu et al.The polyethylene wax-syndiotactic polypropylene formulation of thepresent invention may be used to form monolayer films, but typicallywill be used in multilayer films where the polyethylene-wax syndiotacticpolypropylene film may be used to form a surface layer. For a furtherdescription of the application of the present invention in producingfibers and film products, reference is made to the aforementioned U.S.Pat. Nos. 6,416,699 to Gownder and 6,641,993 to Hanyu et al., the entiredisclosures of which are incorporated herein by reference.

The polyethylene wax syndiotactic polypropylene formulation of thepresent invention can also be employed in various molding operationsthat may take the form of blow molding operations or constricted moldingoperations. In blow molding, the plastic polyethylene-syndiotacticpolypropylene composition is extruded through a suitable feed structureinto a mold cavity of suitable configuration while attendantly supplyingair or another gas so that the plastic syndiotactic polypropyleneformulation is formed to a desired configuration within the moldstructure. Blow molding can be employed to produce hollow products ofany suitable configuration, such as bottles as disclosed in U.S. Pat.No. 6,433,103 to Gunther et al. or automobile fuel tanks as disclosed inU.S. Pat. No. 6,294,235 to Detoumay et al. For a description of blowmolding techniques that may be used in carrying out the presentinvention, reference is made to the aforementioned U.S. Pat. Nos.6,294,235 to Detoumay and 6,433,103 Gunther et al., the entiredisclosures of which are incorporated herein by reference. In producingmolded products by constricted molding the shape of the product isformed by the geometry of the mold without the attendant introduction ofa pressurizing gas.

Having described specific embodiments of the present invention, it willbe understood that modifications thereof may be suggested to thoseskilled in the art, and it is intended to cover all such modificationsas fall within the scope of the appended claims.

1-10. (canceled)
 11. A process for producing a polypropylene productcomprising: (a) providing an ethylene-polypropylene compositioncomprising a mixture of: (i) a predominant amount of a syndiotacticpolypropylene having a designated recrystallization temperature and anoriginal minor melting peak and an original second major melting peak;and (ii) no more than 10 wt. % of a polyethylene wax in admixture withsaid syndiotactic polypropylene in an amount sufficient to provide arecrystallization temperature of said admixture which is greater thanthe designated recrystallization peak of said syndiotactic polypropyleneand a minor melting peak and major melting peak having a temperaturedifference which is lower than the temperature difference between saidoriginal minor melting peak and said original major melting peak; (b)heating said polyethylene-polypropylene composition to a temperature inexcess of the major melting peak of said composition to provide aplastic mass of said polyethylene-polypropylene composition; (c)processing said plastic mass to a desired configuration of said product;and (d) cooling said plastic mass to a temperature below therecrystallization temperature of said admixture to provide said product.12. The process of claim 11 wherein said plastic mass is processed byextruding said plastic mass through a slotted die system to provide apolypropylene film preform which is cooled to a temperature below thecrystallization temperature to produce a film.
 13. The process of claims11 wherein said plastic mass is introduced into a mold and said plasticmass is cooled to a temperature below the recrystallization temperatureto produce a molded product.
 14. The process of claim 11 wherein saidplastic mass is extruded through a fiber die system to produce anelongated fiber which is cooled to a temperature below therecrystallization temperature of plastic mass to form a fiber product.15. The process of claim 11 wherein said polyethylene wax is present insaid formulation in an amount to provide an enhancement in therecrystallization temperature of said formulation which is at least 5%greater than the designated recrystallization temperature of saidsyndiotactic polypropylene without the addition of said polyethylenewax.
 16. The process of claim 11 where the temperature differencebetween the major and minor melting peaks of said formulationincorporating said polyethylene wax is less than the temperaturedifference between the original major and minor peaks without theaddition of said polyethylene wax by an increment of at least 2° C. 17.The process of claim 11 wherein the slope of the curve defined by theoriginal minor melting peak of said syndiotactic polypropylene and theminor melting peak of said admixture of polyethylene wax and saidsyndiotactic polypropylene is greater than the slope of the curvedefined by the original major melting peak of said syndiotacticpolypropylene and the major melting peak of said admixture ofpolyethylene wax and said syndiotactic polypropylene.
 18. The process ofclaim 11 wherein the temperature difference between therecrystallization temperature of said mixture and the minor melting peakof said mixture is less than the difference between said originalrecrystallization temperature and said original minor melting peak by anincrement of at least 1° C.